The present invention relates to a novel compound perfluoro-1-propyl-3,4-dimethylpyrrolidine, a constant boiling composition containing the compound, a process for producing the composition, and uses of the constant boiling composition.
In the process of manufacturing a semiconductor device, liquid perfluorocarbons (hereinafter referred to as xe2x80x9cPFCsxe2x80x9d) having an excellent electric insulating property have been heretofore widely used as the heat medium for controlling the wafer temperature within a chamber in an etching process using a high-voltage plasma or for maintaining a high temperature or a low temperature in the thermal shock test of a wafer or in the performance inspection of the wafer.
Recently, a problem of global warming has been raised and based on the protocol adopted at the Kyoto conference in 1997, Japan has also started to establish goals for reducing the discharge of six kinds of gases including PFCs and to take a specific action.
PFCs are very stable (inactive), both thermally and chemically, and are not considered to deplete the ozone layer because the ozone depleting potential (ODP) thereof is zero due to the absence of chlorine atom in the molecule. Moreover, PFCs are low toxic and noncombustible. By virtue of these excellent properties, they have been widely used, particularly, in the field of manufacture of semiconductor devices where corrosion resistance, insulating resistance and the like are required. Representative examples of liquid PFCs include C5F12, C6F14 and C8F18. These PFCs are, however, known to have a great effect on warming because of their very high stability and the global warming potential (GWP) thereof are as high as 5,000 to 7,000 (integral term: 100 years, assuming that CO2 is 1).
In the case of using these PFCs as the above-described heat medium in an open system or even in a closed system, since complete enclosure is difficult in view of the apparatus design or structure, the system is substantially opened to atmospheric conditions in many cases. Therefore, particularly when the temperature is close to the boiling point of the substance used, the substance inevitably volatilizes into the atmosphere in a large amount. This not only increases the cost necessary for adding and replenishing the heat medium but also raises a serious problem in view of the discharge of a warming gas.
Hydrofluoroethers (hereinafter referred to as xe2x80x9cHFEsxe2x80x9d) and hydrofluorocarbons (hereinafter referred to as xe2x80x9cHFCs), which are considered to have a small effect on warming, have been developed as alternatives of PFCs. A representative example of HFEs is C4F9OC2H2H5 and a representative example of HFCs is cyclic C5H2F8. However, these substances are relatively low in boiling point (max.: about 80xc2x0 C.), and therefore, they cannot be used at high temperatures which reaches 100xc2x0 C., and the electric properties thereof are inferior to conventional PFCs. Because of these reasons, their use in the field of manufacturing of semiconductor devices is limited and these substances are mainly used as a cleaning agent or a solvent.
In the process of manufacturing a semiconductor device, the use form of the heat medium is classified into indirect heat transfer and direct heat transfer. Indirect heat transfer is where the heat medium itself circulates through a heat transfer path (e.g., heat exchanger) and repeats heat absorption or heat release. Direct heat transfer is where the temperature of a material body is changed or maintained while allowing the heat medium to stand in the state of directly contacting with the material body, for example, by dipping the material body directly in the heat medium. In either case, a wide temperature region from an extremely low temperature of about xe2x88x9250xc2x0 C. to a high temperature of 100xc2x0 C. must be continuously or intermittently covered. Therefore, there is a demand for a system of operating the transfer of heat by one kind of heat medium. For example, in the case of a substance which does not boil in a high temperature region (that is, a substance having a high boiling point of at least 100xc2x0 C. or more), the substance is required to have properties of maintaining the liquid phase even in an extremely low temperature region of xe2x88x9250xc2x0 C., which is included in the temperature range during use, and required to have appropriate flowing properties at the same time. For this purpose, the freezing point or the temperature where the substance starts flowing (pour point) is preferably lower than the lowest temperature on use.
In recent years, studies have been made on extremely low temperature conditions on the level of xe2x88x9270xc2x0 C. and therefore, the substance must have the required properties at low temperatures. On the other hand, from the standpoint of coping with the warming problem, the substance must be reduced in volatilization loss by having the required properties at high temperatures. Accordingly, in practice, the range from the freezing point to the boiling point of the heat medium is preferably the temperature range xc2x130xc2x0 C. during use, namely, approximately from xe2x88x92100 to 130xc2x0 C. The existing HFEs and HFCs are known to have a relatively low freezing point but are inferior to PFCs in the electrical properties such as dielectric breakdown voltage and volume resistivity. On the other hand, PFCs are excellent in these electrical properties but those having a low freezing point are liable to have a small molecular weight and a low boiling point. As such, when conventional HFEs, HFCs and PFCs are used as a sole compound, they cannot be a preferred heat medium that is well-balanced in physical properties by practically having a broad temperature range in terms of the temperature range from the freezing point to the boiling point and having good electrical properties at the same time.
Therefore, for example, a method of mixing specific components to provide an azeotropic composition or an azeotrope-like composition to obtain a constant boiling mixture, or a method of adjusting the mixing ratio according to the use conditions to control the properties of the mixture may be developed. However, for using the heat medium in such a system, the constant boiling composition is indispensable, but the combination of existing HFEs, HFCs and PFCs cannot form an azeotropic composition. Moreover, the method of adjusting the mixing ratio highly probably incurs changes in the composition and this is disadvantageous in practice.
The present invention has been made under these circumstances. An object of the present invention is to provide a heat medium, which can contribute to the reduction in the discharge of warming gas by having high boiling properties, can maintain the liquid phase in a wide temperature range during use by having a low freezing point and, at the same time, is favored with excellent electrical insulating properties.
As a result of extensive investigations to solve the above-described problems, the present inventors have found that a constant boiling composition comprising perfluoro-1-propyl-3,4-dimethylpyrrolidine, perfluoro-1-propyl-3-methylpiperidine and perfluorotripropylamine exhibits excellent properties as a heat medium in the process of manufacturing a semiconductor device. Furthermore, extensive investigations have been made on the process for producing the composition. As a result, the present inventors have found that the constant boiling composition can be easily synthesized by using a triallylamine having high general use property and electrolytically fluorinating it in an anhydrous liquid hydrogen fluoride. The present invention has been accomplished based on these findings.
An embodiment of the present invention (I) is a process for producing a perfluorocyclicamine, comprising electrolytically fluorinating a triallylamine in an anhydrous liquid hydrogen fluoride to produce a perfluorocyclicamine represented by the following formula (1): 
wherein xe2x80x94Xxe2x80x94 represents xe2x80x94CF(CF3)xe2x80x94 or xe2x80x94CF2CF2xe2x80x94. In a preferred embodiment, the concentration of triallylamine in the anhydrous liquid hydrogen fluoride is in the range from 1 to 20% by mass and the current density is in the range from 0.1 to 10 A/dm2.
Another embodiment of the present invention (II) is perfluoro-1-propyl-3,4-dimethylpyrrolidine which is a novel compound and which is a compound represented by the formula (1) where xe2x80x94Xxe2x80x94 is xe2x80x94CF(CF3)xe2x80x94.
Another embodiment of the present invention (III) is a process for producing a constant boiling composition comprising cis-perfluoro-1-propyl-3,4-dimethylpyrrolidine, trans-perfluoro-1-propyl-3,4-dimethylpyrrolidine, perfluoro-1propyl-3methylpiperidine and perfluorotripropylamine, the process comprising electrolytically fluorinating a triallylamine in an anhydrous liquid hydrogen fluoride. In a preferred embodiment, the concentration of triallylamine in an anhydrous liquid hydrogen fluoride is in the range from 1 to 20% by mass and the current density is in the range from 0.1 to 10 A/dm2.
Another embodiment of the present invention (IV) is a constant boiling composition comprising cis-perfluoro-1-propyl-3,4-dimethylpyrrolidine, trans-perfluoro-1-propyl-3,4-dimethylpyrrolidine, perfluoro-1-propyl-3-methylpiperidine and perfluorotripropylamine. In a preferred embodiment, the content of perfluorotripropylamine is 55% or less, the boiling point is from 127 to 129xc2x0 C., and the freezing point is xe2x88x92100xc2x0 C. or less.
Other embodiments of the present inventions (V) and (VI) are an electrical insulator using the constant boiling composition of the embodiment of the present invention (IV) and a heat medium using the constant boiling composition of the embodiment of the present invention (IV).
The present invention is described in detail below. With respect to the process for producing perfluoro-tertiary amine, U.S. Pat. No. 2,519,983 (Simons), U.S. Pat. No. 2,567,011 (Simons et al.) and U.S. Pat. No. 2,616,927 (Kauck) describe a process of electrolytically fluorinating a triethylamine in an anhydrous liquid hydrogen fluoride to produce a perfluorotriethylamine. Furthermore, L. Conte et al. report in J. Fluorine Chem., 30, 89 (1985) a process of electrolytically fluorinating a tripropylamine as an alkyl tertiary amine, similarly, in an anhydrous liquid hydrogen fluoride to produce a perfluorotripropylamine (hereinafter referred to as xe2x80x9cPFTPAxe2x80x9d). As such, the perfluoro-tertiary amine has been heretofore produced from a corresponding alkyl tertiary amine.
With respect to the process for producing a perfluorocyclicamine, E. Hayashi et al. report in J. Fluorine Chem., 26, 417 (1984) that when a pipecoline such as N-(3-chloropropyl)-3-pipecoline is electrolytically fluorinated in an anhydrous liquid hydrogen fluoride, a plurality of perfluoropyrrolidine isomers are present in the electrolytic fluorination product.
As such, the process for producing a perfluoroalkyl tertiary amine from a corresponding alkyl tertiary amine by the electrolytic fluorination is known. Also, for obtaining a cyclic amine, a process of producing a perfluorocyclicamine starting from a compound having the objective cyclic structure through electrolytic fluorination is known. However, the process of electrolytically fluorinating a triallylamine which is a tertiary amine having an unsaturated alkyl group, to produce a perfluorocyclicamine of the present invention is heretofore not known and is a novel process.
The present invention has been achieved based on novel knowledge that a triallylamine which is relatively easily available is used as a raw material and as shown in the following formulae (1) and (2), two allyl groups out of three allyl groups in the molecule are combined as propyl radicals during the electrolytic fluorination, thereby cyclizing and isomerizing. 
The embodiment of the present invention (I) is a process for producing cyclic amines, namely, cis-perfluoro-1-propyl-3,4-dimethylpyrrolidine (hereinafter referred to as xe2x80x9ccis-PFPDMPxe2x80x9d), trans-perfluoro-1-propyl-3,4-dimethylpyrrolidine (hereinafter referred to as xe2x80x9ctrans-PFPDMPxe2x80x9d) and perfluoro-1-propyl-3-methylpiperidine (hereinafter referred to as xe2x80x9cPFPMPxe2x80x9d), where the cyclic amines are produced by electrolytically fluorinating a triallylamine in an anhydrous liquid hydrogen fluoride. For this electrolytic fluorination reaction, a known electrolytic method heretofore commonly used in the electrolytic fluorination reaction, such as Simons type electrolytic cell, can be used.
This reaction is performed in an anhydrous liquid hydrogen fluoride and the concentration of the raw material triallylamine in the anhydrous liquid hydrogen fluoride can be selected from the range of 1 to 20% by mass, preferably from 5 to 10% by mass. If the triallylamine concentration exceeds 20% by mass, the yield is liable to decrease, whereas if it is less than 1% by mass, the reaction is liable to proceed slowly and this is not preferred.
The current density can be selected from the range of 0.1 to 10 A/dm2, preferably from 2 to 4 A/dm2. If the current density is too high, the electrolytic power excessively increases and side reactions are disadvantageously liable to take place.
The electrolytic temperature can be selected from the range of xe2x88x9220 to 50xc2x0 C., preferably xe2x88x9210 to 20xc2x0 C. If the reaction temperature is too low, the electrolytic voltage is liable to excessively increase, whereas if it is too high, hydrogen fluoride is readily volatilized and this is not preferred.
The electrolysis reaction can be usually performed under an atmospheric pressure but if desired, may be performed under pressure. In the case where the reaction is performed under pressure, the boiling point of hydrogen fluoride elevates and cooling of the reaction system can be advantageously mitigated.
In order to perform the electrolytic fluorination with good efficiency and improve the yield of the objective product, the electrolytic solution is preferably stirred and mixed during the reaction and for this purpose, means such as mechanical forced stirring or stirring by introduction of an inert gas may be used.
The thus-obtained perfluorocyclicamine contains, in addition to the objective product, low molecular substances having a low boiling point produced due to the cleavage during the fluorination or polymer substances produced due to the recombination. The high boiling materials including the objective product have a specific gravity higher than that of hydrogen fluoride and mostly remain on the inner bottom of the electrolytic cell. This deposit does not dissolve in the liquid hydrogen fluoride but is separated to form two layers. Therefore, the lower layer can be separated and taken out after the completion of electrolysis.
The solution of the lower layer separated and discharged is washed by a known method using an aqueous caustic soda solution or the like and after neutralizing and removing the hydrogen fluoride, dried to obtain a crude product. From this crude product, at least three kinds of perfluorocyclicamine isomers can be finally isolated by an isolation method using fractional gas chromatography or distillation, preferably precision distillation. By performing the structural analysis, each component can be confirmed to have a structure so that the raw material triallylamine is cyclized and thereby isomerized, as described above, to convert into a perfluorocyclicamine, namely, cis-PFPDMP, trans-PFPDMP and PFPMP. Then, this mixture of components can be separated into respective components by a precision distillation through increased plates or a method described later.
The embodiment of the present invention (II) is perfluoro-1-propyl-3,4-dimethylpyrrolidine (PFPDMP) and can be obtained by the production process of the present invention (I). This compound is a novel compound and is represented by formula (1): 
where xe2x80x94Xxe2x80x94 is xe2x80x94CF(CF3)xe2x80x94. This compound has stereoisomers and includes cis-3,4-trifluoromethyl form (cis-PFPDMP) and trans-3,4-trifluoromethyl form (trans-PFPDMP).
The embodiment of the present invention (III) is a process for producing a constant boiling composition comprising cis-perfluoro-1-propyl-3,4-dimethylpyrrolidine (cis-PFPDMP), trans-perfluoro-1-propyl-3,4-dimethylpyrrolidine (trans-PFPDMP), perfluoro-1-propyl-3-methylpiperidine (PFPMP) and perfluorotripropylamine (PFTPA) by the electrolytic fluorination reaction of a triallylamine.
The conditions for the electrolytic fluorination may be the same as the conditions described above with respect to the production of perfluorocyclicamine. The discharged solution of the lower layer obtained after the electrolysis reaction is washed by a known method using an aqueous caustic soda solution or the like and after neutralizing and removing the hydrogen fluoride, dried to obtain a crude product. The thus-obtained crude product contains PFTPA in addition to the above-described cis-PFPDMP, trans-PFPDMP and PFPMP. PFTPA is a resultant from the fluorination and conversion of a part of the raw material triallylamine into PFTPA while not allowing the allyl groups to combine to form a ring. According to the production process of the present invention, a mixed composition mainly comprising four kinds of perfluoro-tertiary amines (including cyclic amine) can be obtained.
These four kinds of perfluoro-tertiary amines contained in this mixed composition have similar boiling points and, in general, separation into respective single components is difficult in industry. Therefore, in the embodiment of the present invention (IV), the mixed composition mainly comprising the above-described four kinds of perfluoro-tertiary amines is collected as an intermediate fraction, for example, by a method of cutting unnecessary low boiling and high boiling portions through simple and convenient batch distillation, and the mixture is used as it is. The four kinds of perfluoro-tertiary amines are similar in boiling point and therefore, need not be particularly separated into single components. By this process, a mixed composition having a purity of 99% or more (GC area) in terms of the purity of four components can be obtained. The purity is not particularly limited. The impurities contained other than those main components are fundamentally perfluoro-tertiary amines having a similar cyclic structure and have the same properties. Therefore, even if these are intermingled in slight amounts, no problem arises. As long as at least the main components are contained in a concentration of 95% or more, the effect of the present invention can be attained. The conditions for the GC analysis may be general conditions used in the analysis of fluorine compounds.
When the boiling point of the mixed composition obtained by the above-described process is examined while variously changing the composition, the boiling point is scarcely changed depending on the composition and the mixed composition behaves as a constant boiling composition of 127 to 129xc2x0 C. Accordingly, with respect to the boiling point of the mixed composition, the compositional ratio of four components is not particularly limited. In a preferred embodiment of the mixed composition of the present invention (IV), the freezing point is xe2x88x92100xc2x0 C. or less and the compositional ratio of PFTPA is 55% or less. The compositional ratio of PFTPA can be determined from the GC area percentage.
The constant boiling composition of the present invention (IV) is not limited to the mixed composition obtained by the above-described production process but may be a composition prepared, for example, by mixing cis-PFPDMP, trans-PFPDMP, PFPMP and PFTPA which are separately isolated.
In either case, a constant boiling composition in the vicinity of 128xc2x0 C., that is, an azeotrope like mixture is fundamentally formed. Since the compositions of the gas phase and the liquid phase are almost the same at the boiling point, the change in the composition accompanying the volatilization during the use can be neglected and the physical properties are mostly free of fear for changes. Thus, the composition is very useful in practice.
The constant boiling composition of the present invention (IV), which contains perfluorocyclicamines and PFTPA and has a freezing point of about xe2x88x92100xc2x0 C. or less and a boiling point of 127 to 129xc2x0 C., exhibits in any case a dielectric breakdown voltage of 45 kV or more and a volume resistivity on the order of 1015 to 1016. In addition, the composition is comparable to conventional heat mediums comprising PFCs and has more excellent characteristics than HFEs. The embodiment of the present invention (V) is a use of the constant boiling composition of the present invention (IV) as an electrical insulator. This electrical insulator can be used not only in the field of manufacture of semiconductors but also as an alternative of usual insulating oils by sealing it in a transformer, an electric power source breaker, a capacitor or the like.
The constant boiling composition of the present invention, which contains perfluorocyclicamines and PFTPA and has a freezing point of about xe2x88x92100xc2x0 C. or less and a boiling point of 127 to 129xc2x0 C., maintains the liquid phase in the temperature range during use of xe2x88x9270 to 100xc2x0 C., for example, under an atmospheric pressure. Accordingly, the embodiment of the present invention (VI) is a use of the constant boiling composition of the present invention (IV) as a heat medium. The role of this heat medium is not limited only to circulate by itself through a heat transfer path and to indirectly heat or cool a material to which the heat is transmitted, but also includes directly contacting a material to which the heat is transmitted and heating or cooling the material. Furthermore, the temperature range during use is not limited to the range from xe2x88x9270 to 100xc2x0 C., and the heat medium may be used under higher temperature conditions by completely closing the system and maintaining the liquid state under pressure of atmospheric pressure or more.